1. Field of the Invention
The present invention relates to superconducting quantum interference devices (SQUIDs) and, more particularly, to a flux-locked loop circuit with an expanded operating range of a SQUID.
2. Description of the Related Art
A superconducting quantum interference device (hereinafter referred to as “SQUID”) is a device for detecting minute magnetic fields. The SQUID is manufactured using a superconducting material to operate in an extremely low temperature helium or liquid nitrogen Dewar and converts a magnetic field into a voltage. In order to enhance measurement sensitivity and reduce peripheral environment noise, the SQUID is combined with various pick-up coils to be used as a precise magnetic sensor.